[Outliner] Add an option to only enable outlining of patterns above a…

… certain threshold

Outlining isn't always a win when the saved instruction count is >= 1.
The overhead of representing a new function in the binary depends on
exception metadata and alignment. So parameterize this for local tuning.

Reviewed By: paquette

Differential Revision: https://reviews.llvm.org/D136774

llvm/lib/CodeGen/MachineOutliner.cpp

@@ -116,6 +116,11 @@ static cl::opt<unsigned> OutlinerReruns(
     cl::desc(
         "Number of times to rerun the outliner after the initial outline"));
 
+static cl::opt<unsigned> OutlinerBenefitThreshold(
+    "outliner-benefit-threshold", cl::init(1), cl::Hidden,
+    cl::desc(
+        "The minimum size in bytes before an outlining candidate is accepted"));
+
 namespace {
 
 /// Maps \p MachineInstrs to unsigned integers and stores the mappings.
@@ -664,7 +669,7 @@ void MachineOutliner::findCandidates(
       continue;
 
     // Is it better to outline this candidate than not?
-    if (OF->getBenefit() < 1) {
+    if (OF->getBenefit() < OutlinerBenefitThreshold) {
       emitNotOutliningCheaperRemark(StringLen, CandidatesForRepeatedSeq, *OF);
       continue;
     }
@@ -840,7 +845,7 @@ bool MachineOutliner::outline(Module &M,
     });
 
     // If we made it unbeneficial to outline this function, skip it.
-    if (OF.getBenefit() < 1)
+    if (OF.getBenefit() < OutlinerBenefitThreshold)
       continue;
 
     // It's beneficial. Create the function and outline its sequence's

llvm/test/CodeGen/AArch64/machine-outliner-threshold.mir

@@ -0,0 +1,114 @@
+# RUN: llc -mtriple=aarch64--- -run-pass=machine-outliner -verify-machineinstrs %s -o - | FileCheck %s
+# RUN: llc -mtriple=aarch64--- -outliner-benefit-threshold=10 -run-pass=machine-outliner -verify-machineinstrs %s -o - | FileCheck %s -check-prefix=THRESHOLD
+--- |
+
+  define void @baz() #0 {
+    ret void
+  }
+
+  define void @bar(i32 %a) #0 {
+    ret void
+  }
+
+  attributes #0 = { noredzone }
+...
+---
+# Check that two we outline two different sequences, one from bb1 and one from
+# bb2 when the threshold is 1.
+# CHECK-LABEL: bb.1:
+# CHECK: BL @OUTLINED_FUNCTION_[[F0:[0-9]+]], implicit-def $lr, implicit $sp
+# CHECK-NEXT: $w11 = ORRWri $wzr, 1
+# CHECK-NEXT: $w11 = ORRWri $wzr, 1
+# CHECK-NEXT: $w11 = ORRWri $wzr, 2
+# CHECK-NEXT: BL @baz, implicit-def dead $lr, implicit $sp
+# CHECK-NEXT: BL @OUTLINED_FUNCTION_[[F0]], implicit-def $lr, implicit $sp
+# CHECK-NEXT: $w11 = ORRWri $wzr, 1
+# CHECK-NEXT: $w11 = ORRWri $wzr, 1
+# CHECK-NEXT: $w8 = ORRWri $wzr, 0
+# CHECK-NOT: $w11 = KILL renamable $w11, implicit killed $w11
+
+# CHECK-LABEL: bb.2:
+# CHECK: BL @OUTLINED_FUNCTION_[[F1:[0-9]+]], implicit-def $lr, implicit $sp
+# CHECK-NEXT: $w9 = ORRWri $wzr, 0
+# CHECK-NEXT: BL @OUTLINED_FUNCTION_[[F1]], implicit-def $lr, implicit $sp
+# CHECK-NEXT: $w8 = ORRWri $wzr, 0
+# CHECK-NOT: $w11 = KILL renamable $w11, implicit killed $w11
+
+# Check that the sequences in bb.2 don't get outlined with a threshold of 10 but
+# the sequences in bb.1 do.
+# THRESHOLD-LABEL: bb.1:
+# THRESHOLD: BL @OUTLINED_FUNCTION_[[F0:[0-9]+]], implicit-def $lr, implicit $sp
+# THRESHOLD-NEXT: $w11 = ORRWri $wzr, 1
+# THRESHOLD-NEXT: $w11 = ORRWri $wzr, 1
+# THRESHOLD-NEXT: $w11 = ORRWri $wzr, 2
+# THRESHOLD-NEXT: BL @baz, implicit-def dead $lr, implicit $sp
+# THRESHOLD-NEXT: BL @OUTLINED_FUNCTION_[[F0]], implicit-def $lr, implicit $sp
+# THRESHOLD-NEXT: $w11 = ORRWri $wzr, 1
+# THRESHOLD-NEXT: $w11 = ORRWri $wzr, 1
+# THRESHOLD-NEXT: $w8 = ORRWri $wzr, 0
+# THRESHOLD-NOT: $w11 = KILL renamable $w11, implicit killed $w11
+
+# THRESHOLD-LABEL: bb.2:
+# THRESHOLD-NOT: BL @OUTLINED_FUNCTION
+name:            bar
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $lr, $w8
+    $sp = frame-setup SUBXri $sp, 32, 0
+    $fp = frame-setup ADDXri $sp, 16, 0
+
+  bb.1:
+    BL @baz, implicit-def dead $lr, implicit $sp
+    $w11 = ORRWri $wzr, 1
+    $w11 = ORRWri $wzr, 1
+    $w11 = KILL renamable $w11, implicit killed $w11
+    $w11 = ORRWri $wzr, 1
+    $w11 = ORRWri $wzr, 1
+    BL @baz, implicit-def dead $lr, implicit $sp
+    $w11 = ORRWri $wzr, 1
+    $w11 = ORRWri $wzr, 1
+    $w11 = ORRWri $wzr, 2
+    BL @baz, implicit-def dead $lr, implicit $sp
+    $w11 = ORRWri $wzr, 1
+    $w11 = ORRWri $wzr, 1
+    $w11 = ORRWri $wzr, 1
+    $w11 = ORRWri $wzr, 1
+    BL @baz, implicit-def dead $lr, implicit $sp
+    $w11 = ORRWri $wzr, 1
+    $w11 = ORRWri $wzr, 1
+    $w8 = ORRWri $wzr, 0
+
+  bb.2:
+    $w15 = ORRWri $wzr, 1
+    $w15 = ORRWri $wzr, 1
+    $w15 = ORRWri $wzr, 1
+    $w15 = ORRWri $wzr, 1
+    $x15 = ADDXri $sp, 48, 0;
+    $w9 = ORRWri $wzr, 0
+    $w15 = ORRWri $wzr, 1
+    $w15 = ORRWri $wzr, 1
+    $w15 = ORRWri $wzr, 1
+    $w15 = ORRWri $wzr, 1
+    $x15 = ADDXri $sp, 48, 0;
+    $w8 = ORRWri $wzr, 0
+
+  bb.3:
+    $fp, $lr = LDPXi $sp, 2
+    $sp = ADDXri $sp, 32, 0
+    RET undef $lr
+
+...
+---
+name:            baz
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $lr, $w8
+    RET undef $lr
+
+# CHECK-LABEL: name:            OUTLINED_FUNCTION_{{[0-9]}}
+# CHECK-LABEL: name:            OUTLINED_FUNCTION_{{[1-9]}}
+
+# THRESHOLD-LABEL: name:            OUTLINED_FUNCTION_{{[0-9]}}
+# THRESHOLD-NOT:   name:            OUTLINED_FUNCTION_{{[1-9]}}